Fabrication of macroporous silicon

ABSTRACT

A process for the fabrication of macroporous silicon from silicon wafers. Preferred embodiments include a two-step-etch process that results in a single, macroporous layer of silicon with pore depths of several microns and relatively uniform equivalent diameters with an operator chosen mean equivalent diameter. The mean diameter determined by the operator is a mean diameter within the range of about 40 nm to about 250 nm and is determined, at least in part, by selection of an applied current density. Uniformity of equivalent pore diameter is greatly improved as compared to prior art porous silicon fabrication techniques. In a first electrochemical anodisation step, a macroporous layer is created that is covered by a shallower combination microporous-mesoporous layer. The wafer is removed and the top surface of the wafer is dissolved under alkaline conditions, producing “pits”. These “pits” serve as defect sites during a second electrochemical anodisation step, resulting in a single, uniform, macroporous layer in the silicon wafer. Preferably, the wafer with its porous silicon surface is quickly rinsed in acetone and then pentane to preserve the structure of the pores and prevent collapsing of pore walls. The process of the present invention eliminates the upper, nanoporous region produced by prior art processes. In a preferred embodiment, utilizing a current density of 181.8 mA/cm 2  in the two-etch-step process, the resulting mean equivalent pore diameter is about 100 nm with more than half of the equivalent pores diameters within about ±50 nm of the mean equivalent pore diameter. By increasing or decreasing the anodisation current, porous layers with mean diameters within the range of about 40 nm to about 250 nm can be created.

This invention relates to processes for the fabrication of poroussilicon products and in particular to techniques for the fabrication ofsuch products with average equivalent pore diameters in the range ofabout 40 to 250 nanometers.

BACKGROUND OF THE INVENTION Porous Silicon

Porous silicon parts are produced by electrochemically etching siliconwafers in a hydrofluoric acid bath, which yields porous layers on one ofthe surfaces of the wafer. The first experiments at producing theselayers were more than 40 years ago. In the etching process, siliconatoms are dissolved from the bulk material forming tiny pores and theoriginal crystal structure remains unaffected. Dependent on theexperimental conditions (doping density of bulk material, electrolyteconcentration, current density, etching voltage, temperature) differentmodifications of porous silicon can be formed. With the different poreetching techniques it is possible to vary pore sizes in the silicon on ascale from millimeters to nanometers. A very large number of technicalapplications of porous silicon have been proposed. These include opticalcomponents, electronic components and electro-optical components.

In a typical prior art process, a silicon wafer is immersed in anethanolic hydrofluoric acid solution between a platinum cathode and aplatinum anode and a constant electric current is applied to the wafer.The silicon atoms at the silicon/electrolyte interface facing thecathode are polarized, and are subject to attack by the fluoride ions insolution. Silicon atoms are released in the form of siliconhexafluoride. Porous silicon tends to etch as a distribution ofapproximately cylindrical pores with very small diameters that tend tobe much deeper than they are wide. The approximately cylindrical shapeof the pores and their depths can be amazingly uniform. The distributionof pore diameters and the depth of the pores are controlled by adjustingthe current density and the etching time. Additional details relating tothese processes are contained in U.S. Pat. No. 6,248,539 that isincorporated herein by reference.

The International Union of Pure and Applied Chemistry (IUPAC) guidelinesdefine ranges of pore sizes as shown in Table 1 below: TABLE 1 IUPACClassification of Pore Size. Pore Width (nm) Type of Pore  <2 micro 2-50meso >50 macro

Single Step Fabrication Results

Prior art fabrication techniques to produce porous silicon partsincluded a single-step, electrochemical dissolution of veryhighly-doped, p-type porous silicon. The disadvantages of this methodare:

-   -   1) requirements for a very limited wafer-resistivity range        (ρ=0.00065-0.00075 Ωcm);    -   2) difficulty and cost associated with obtaining such wafers;        and    -   3) uncertain results.

In addition, SEM imaging of the etched silicon revealed an upper shallowlayer of micropores and mesopores (with equivalent diameters generallymuch less than 50 nanometers) covering a mixed, macroporous layer (adeeper layer of pores having pores with equivalent diameters generallygreater than 50 nanometers). The upper layer appeared to vary in depthfrom several hundred nanometers to several microns.

What is needed is a better method for making porous silicon productswith uniform average pore diameters in the range of about 50 nanometersto about 250 or greater.

SUMMARY OF THE INVENTION

This invention provides a process for the fabrication of macroporoussilicon from silicon wafers. Preferred embodiments include atwo-etch-step process that results in a single, macroporous layer ofsilicon with pore depths of several microns and relatively uniformequivalent diameters with an operator chosen mean equivalent diameter.The mean diameter determined by the operator is a mean diameter withinthe range of about 40 nm to about 250 nm and is determined at least inpart by a selection of an applied current density. Uniformity ofequivalent pore diameter is greatly improved as compared to prior artporous silicon fabrication techniques. In a first electrochemicalanodisation step a macroporous layer is created that is covered by ashallower combination microporous-mesoporous layer. The wafer is removedand the top surface of the wafer is dissolved under alkaline conditions,producing “pits”. These “pits” serve as defect sites during a secondelectrochemical anodisation step, resulting in a single, uniformmacroporous layer in the silicon wafer. Preferably, the wafer with itsporous silicon surface is quickly rinsed in acetone and then pentane topreserve the structure of the pores and prevent collapsing of porewalls. The process of the present invention eliminates the upper,nanoporous region produced by prior art processes. In a preferredembodiment, utilizing a current density of 181.8 mA/cm² in thetwo-etch-step process, the resulting mean equivalent pore diameter isabout 100 nm with more than half of the equivalent pores diameterswithin about ±50 nm of the mean equivalent pore diameter. By increasingor decreasing the current porous layers with mean diameters within therange of about 40 mu to about 250 nm can be created.

In addition to the elimination of the upper, nanoporous layer, the newmethod also has the advantages of:

-   -   1) utilizing lower concentrations of hydrofluoric acid, thereby        decreasing safety hazards;    -   2) utilizing silicon wafers with wider, less expensive and more        readily available resistivity ranges (ρ=0.001-0.0035 Ω-cm); and    -   3) increasing anodisation times to allow for better control and        better reproducibility. In addition, the process according to        the present invention permits the reproducible fabrication of        porous silicon products with relatively uniform equivalent pore        diameters with average equivalent pore diameters ranging from        about 50 nm to about 250 nm with defined depths up to several        microns. These varieties of average pore diameters and depths        are produced with the proper choice of anodisation current        density and etch time.

In preferred embodiments for producing wafers for use in a molecularsensor, the porous structure is surface modified by molecular vapordeposition of silane compounds to increase wettability, stability andconfer functionality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron micrograph of electrochemically etchedporous silicon that results from the prior art utilizing p-type silicon(ρ=0.00065 Ω-cm).

FIG. 2 is a scanning electron micrograph of electrochemically etchedporous silicon utilizing lower-resistivity, more readily available,p-type silicon (ρ=0.0011 Ω-cm).

FIG. 3 is a scanning electron micrograph of electrochemically etchedporous silicon utilizing lower-resistivity, more readily available,p-type silicon (ρ=0.0011 Ω-cm), post KOH-dissolution.

FIG. 4A is a scanning electron micrograph (cross-sectional view) of anupside down electrochemically etched porous silicon using the newmethod.

FIG. 4B is a scanning electron micrograph (tilted top view) ofelectrochemically etched porous silicon using the new, double-etchmethod.

FIGS. 5A through 5E are a representative sampling of different scanningelectron micrographs (top views) of electrochemically etched poroussilicon, demonstrating some of the different pore size distributionsachievable with this invention and showing the effect on pore size ofincreasing current.

FIGS. 6A and 6B show important components of an anodisation cell.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Fabrication of PorousSilicon with Average Pore Diameters of 100 nm

Described below by reference to the drawings is a preferred process forfabricating porous silicon with an average equivalent pore diameter ofabout 100 nanometers with more than half of the pores having equivalentpore diameters within about ±20 nm of the average 100 nm equivalentdiameter.

In this specification, we will use the phrase, “equivalent porediameter” D_(e), of a pore to refer to the approximate diameter of acomparable circular cylinder having the same volume as that of the pore.Since the cross sectional area of each pore is typically approximatelyuniform along the depth of the pore, we can estimate this equivalentpore diameter by measuring the area, A, of the pore at the surface ofthe wafer and calculating a value for D_(e) as follows: D_(e)=2√{squareroot over (A/π)}.

A preferred anodization cell 48 is shown in an exploded view in FIG. 6Aand in a perspective view in FIG. 6B. It includes cell reservoir 48,wafer holder 50, anode 52 and cathode 54. Wafer holder 50 includes twofluoroelastomer gaskets 60 (such as Viton® gaskets, available fromProblem Solving Products, Inc. with offices in Denver, Colo.) thatprovide seals separating the cell reservoir into an anode region and acathode region to create what is known as a “double-tank” cell.Substantially the entire voltage drop in the cell's electrical circuitis through silicon die 56. (Die 56 is, as indicated in FIG. 6A, a 10mm×13 mm section of a silicon wafer. Although the die section is only asmall part of a wafer it is sometime referred to, itself, as a wafer.)Wafer holder includes two Teflon masks with etching windows 58, eachdefining an etch area of 0.495 cm².

The porous silicon regions are high surface area regions consisting ofnanometer size pores in a crystalline silicon substrate. The pores areproduced by anodic electrochemical etches of bulk crystalline silicon.The starting material for porous silicon, for this preferred embodiment,is a heavily doped crystalline silicon wafer, commercially available forsemiconductor manufacturing purposes. Wafer specifications for thisporous silicon fabrication process include p-type boron doped silicon(0.001-0.0035 Ω-cm resistivity) with a <100> crystal orientation. Fourinch diameter, p-type silicon (100) wafers with resistivity rangesbetween 0.0010 and 0.0035 Ω-cm were purchased from Silicon QuestInternational, Inc., with offices in Santa Clara, Calif.). The waferswere pre-scribed into 44 individual die sections measuring 10 mm×13 mmby American Precision Dicing (San Jose, Calif.) which section, asindicated above, are referred to as dies, die section or wafers. Theactual etch area, defined by the Teflon masks, measures 9.0 mm×5.5 mmand equals 49.5 mm².

All chemicals used were reagent grade or higher and purchased fromHawaii Chemical & Scientific unless otherwise noted. Ultra pure waterwas obtained from a Bamstead Nanopure Diamond Analytical Water System(APC Water Services, Inc.).

Precleaning

Immediately prior to anodisation, wafers were pre-cleaned as describedin this section. Silicon wafer 56 was placed in 40 ml of concentratedsulfuric acid and heated to about 90 degrees C. Twenty milliliters ofhydrogen peroxide (30%) was added to the acid and the wafer was allowedto oxidize for 10 minutes in the heated solution, after which the waferwas rinsed with copious amounts of ultra pure water for 5 minutes. Therinsed silicon wafer was transferred to a clean, glass beaker containing150 ml of ultra pure water and 30 ml of ammonium hydroxide (30%). Thesolution was heated and, once it reached 70 degrees C., 30 ml ofhydrogen peroxide (30%) was added. The silicon wafer remained in thesolution for 15 minutes and again was rinsed with copious amounts ofwater for 5 minutes. A resulting oxide layer was stripped by soaking thewafer in a 2.5% solution of hydrofluoric acid (diluted with water) for 2minutes and again rinsed with copious amounts of water for 5 minutes.The silicon wafer was then transferred to a clean, glass beakercontaining 120 ml of ultra pure water and 30 ml of hydrochloric acid(37%). The solution was heated to 70 degrees, at which time 30 ml ofhydrogen peroxide was added. The silicon wafer remained in the solutionfor 15 minutes before a final five minute rinse with copious amounts ofultra pure water. The wafer was blown dry under an inert stream ofnitrogen gas using a nitrogen source available from GasPro, with officesin Kahului, Hi.

Anodisation

The clean wafer was then assembled into the Teflon etch chamber ofanodisation cell 48 and immersed in an ethanolic hydrofluoric acidsolution. The solution is a mixture of equal quantities of (1) 50%hydrofluoric acid (equal volumes of hydrofluoric acid and water) and (2)ethanol. Applicants refer to this solution as 25 percent hydrofluoricacid in ethanol. Specifically, 40 milliliters of 25 percent hydrofluoricacid in ethanol is slowly added to the cell reservoir. Conductors from apower supply (not shown) are connected to the platinum wire electrodepaddles in the anodisation cell and a constant current density (J=181.8mA/cm²) is applied for 30 seconds. The total area to be etched, definedby windows 58, is 0.99 cm².

Therefore, the appropriate anodisation current is 180 mA. The siliconatoms at the silicon/electrolyte interface are attacked by the fluorideions in solution forming silicon hexafluoride. Silicon atoms arereleased from the wafer in the form of silicon hexafluoride. The etchedsilicon wafer is removed from the anodisation cell, rinsed in acetone,then pentane and allowed to air dry. As shown in FIG. 1 and FIG. 2, theporous silicon that results from this first etch step is bi-layered,with an upper, microporous-mesoporous layer (with equivalent porediameters mostly at about 10 to 50 nanometers) covering a lower,macroporous layer with diameters in the range of about 100 nanometers.(The magnification of the images shown in FIGS. 1 and 2 are indicated bya 1 micron reference line at the bottom of FIG. 1 and a 100 nm referenceline at the bottom of FIG. 2. The upper, microporous-mesoporous layerand the top portion of the lower layer (approximately the top 70 to 90percent of the lower layer) are dissolved in 0.1M KOH, rinsed and driedunder a stream of nitrogen. The remainder of the lower layer appears asrelatively shallow “pits”. These remaining pits, shown in FIG. 3, serveas defect sites for the initiation of a second electrochemical etch.(Note the 100 nm reference line at the bottom of FIG. 3.) The silicon isagain immersed in an ethanolic hydrofluoric acid solution (HF:ethanol,1(v):1(v)) in cell 48 and a constant electric current applied using theplatinum electrodes. The silicon is again anodized at 180 mA (currentdensity=181.8 mA/cm²) for 30 seconds, rinsed in acetone and finally inpentane to prevent collapsing of the pore walls due to high interfacialsurface tension during drying of the porous silicon. The samples areblown dry under an inert stream of nitrogen gas and stored in adessicator for further surface modification.

The result of the above process is a silicon wafer part with a veryuniform, single, macroporous layer as shown in FIGS. 4, 4A and 4B. Thepores are about two microns deep with good symmetry throughout the depthof the pores and very little (less than about 10 percent) variation indepth. The pores are roughly circular but can have final shapes similarto squares, pentagons and hexagons (with narrow walls). About 90 percentof the wafer surface is covered with pores that have “diameters” in therange of about 50 nm to 250 nm but most of the pores have equivalentpore diameters in the range of 100±50 nm.

FIG. 4A is an inverted side cross section image before a gold coating isapplied and FIG. 4B is a tilted top view image after gold coating. FIG.4A confirms the uniformity of pore width as a function of pore depth.

Varying the Pore Diameter and the Depth

The distribution of pore diameters and the depth of the pores may becontrolled by adjusting current density and anodisation duration, asshown in FIGS. 5A through 5E. Typical average pore features forpreferred embodiments produce average equivalent pore diameterdistributions of about 50 to 250 nanometers and pore depths of about2000 to 3000 nanometers. The current densities, J, applied to producethe samples shown in FIGS. 5A through 5E varied from J=162 mA/cm² toJ=404 mA/cm². The pore diameters increase with increasing currentdensity with J=162 mA/cm² creating pores with diameters averaging about40 nm and J=404 mA/cm² producing pores with diameters averaging about250 nm. The depth of the pores is very uniform. This high uniformity ofthe etching process provides the two optically flat interfaces; the topsurface of the porous silicon, and the interface between the bottom ofthe porous silicon region and the non-porous, or bulk, silicon. The poredepth is controlled by the duration of etch.

Surface Modification of Porous Silicon

In preferred embodiments, the porous silicon surface may be modified forparticular applications. In one application the porous silicon isutilized in a molecular sensor to anchor molecules for the purpose ofmonitoring molecular interactions. For this embodiment, after the poroussilicon layer has been produced on the silicon wafer as explained above,a protective layer is applied to prevent or minimize oxidation andcontamination with particulates from ambient air. Preferably the wafersare immediately surface modified or stored under a blanket of inertnitrogen gas in a controlled humidity environment to be surface modifiedlater. Surface modifications can be achieved using a variety oftechniques including wet chemistry and molecular vapor deposition (MVD).Applicants' first preferred embodiment for surface modification relieson MVD technology. MVD overcomes many limitations associated with wetchemistry including cost, process complexity and surface coverage. Theprocess consists of pre-cleaning using argon or oxygen plasma followedby tunable deposition of a monolayer film under sub-atmosphericpressure.

A wide variety of chemicals can be deposited on the surface dependingupon the ultimate application. For a preferred embodiment in which theporous silicon dies are to be used as a molecular sensor for measuringbinding interactions, Applicants describe below the deposition of10-(carbomethoxy)decydimethylchlorosilane (Gelest, Inc.) using amolecular vapor deposition unit Model MVD-100 available from AppliedMicrostructures Inc. with offices in San Jose, Calif. Post-etching,samples were placed in the MVD-100 and cleaned of any organiccontamination by an oxygen plasma treatment, in this case, for 90seconds with a chamber pressure of 0.5 Torr and RF power in the range of100-300 watts. The plasma treatment serves a dual purpose, not onlyeliminating the etched surface of contaminants, but also uniformlyhydroxylating the silicon surface with OH-groups for subsequentsilanization. The organic linker[10-(carbomethoxy)decyldimethylchlorosilane] (Gelest, Inc.) wasvaporized before metered delivery of approximately 2.0-3.0 microlitersto the reaction chamber where it reacted with the hydroxylated siliconsurface in the presence of trace amounts of water, resulting in therelease of a negligible amount of HCL gas and the functionalized siliconsurface. In this case, the vapor was allowed to react for 25-30 minutes.The dies can be used, as is, to couple proteins via standard aminecoupling techniques or further modified with different bioconjugates toincrease hydrophilicity and/or create specific functionalized surfaces.Using this preferred embodiment, Applicants and thier fellow workershave coupled Amino-dPEG₁₂™-t-butyl ester (Quanta Biodesign) to thesurface by first activating the carbomethoxy group of the siliconsurface with 200 mM EDC [1-Ethyl-3-(3-Dimethylaminopropyl)carbodiimideHydrochloride] (Pierce Biotechnology) and 50 mM NHS[N-Hydroxysuccinimide] (Pierce Biotechnology) in water for 10 minutes.The activated surface is then allowed to react with 1 mg/ml ofAmino-dPEG₁₂™-t-butyl ester for 30 minutes and any remaining NHS estersare capped with 1M ethanolamine, pH 8.0 for 10 minutes. The surface isrinsed in ultra pure water, pure ethanol and dried under a stream ofinert nitrogen gas. The final product is a pegylated, porous siliconsurface with a protected carboxylic acid functional group. Thefunctional group may be deprotected by exposure to 25% trifluoroaceticacid (TFA) in ice cold methylene chloride (CH₂Cl₂) for 5 hrs and usedfor immobilization with standard amine coupling techniques.Alternatively, the deprotection step may be avoided by coupling theAmino-dPEG₁₂ ™ acid (Quanta Biodesign) instead of theAmino-dPEG₁₂™-t-butyl ester. In this case, the end user can proceed withactivation and immobilization of the target using EDC/NHS and standardamine coupling. The end product is a functionalized, hydrophilic poroussilicon die with cylindrical, straw-like pores of 100 nm diameters and 2μ depths and two optically flat, parallel surfaces resulting from thetop (air/porous silicon) and bottom (porous silicon/bulk silicon)surfaces of the porous silicon matrix. The structural morphology of thedies provides a convenient two-beam interferometer while the highsurface area and adaptable surface chemistry provide the platform fornumerous protein and DNA sensing applications.

While the present invention is described in terms of preferredembodiments, the reader should understand that these are merely examplesand that many other embodiments are changes to the above embodimentswill be obvious to persons skilled in this art. For example, the size,shape and number of pores in the porous silicon regions could varygreatly depending on the particular application of the presentinvention. Applicants and thier fellow workers have been able to achievereproducible pore sizes with diameters as small as 20 nm and as large asseveral microns. The porosity of the regions may vary greatly with theapplication and many other porosity values could be utilized. Also, theself assembled monolayer and secondary linkers can take limitless forms,depending upon the end user's ultimate application. For instance, ashort pegylated molecule with a hydroxyl group on one end and aprotected acid on the other could be deposited directly onto the poroussilicon surface after plasma treatment using the MVD-100. Linker withfree thiol groups (versus the acid described in the preferredembodiment) can be utilized if the ultimate goal is to immobilizetargets via the sulfhydryl group. Therefore, the scope of the inventionshould be determined by the claims and their legal equivalents.

1. A process for the fabrication of porous silicon from silicon waferswith pore depths of several microns and average equivalent porediameters of about 40 nm to about 250 nm comprising the steps of: A) ina double-tank etch chamber, comprising a positive electrode and negativeelectrode and an etch solution, immobilizing a silicon wafer betweensaid positive and negative electrodes, B) initiating a firstelectrochemical anodisation step to etch on a surface of said siliconstructure a macroporous layer that is covered by a shallower combinationmicroporous-mesoporous layer, C) dissolving the combinationmicroporous-mesoporous layer and most of the mesoporous layer leavingpits in the surface of said silicon structure, and D) initiating asecond electrochemical anodisation step to etch on the surface of saidsilicon structure a mesoporous layer with said pits serving as defectsites to define locations of pores having average diameters of about 40nm to about 250 nm.
 2. The process as in claim 1 wherein said averagediameter of between 40 nm and 250 nm is determined by choice of currentdensity applied in said first and second anodisation steps.
 3. Theprocess of claim 1 wherein said dissolving step is accomplished underalkaline conditions.
 4. The process as in claim 1 and further comprisingthe step of rinsing said silicon structure in acetone and then pentaneprior to initiating said second anodisation step.
 5. The process as inclaim 1 and further comprising a step of modifying the porous surface bymolecular vapor deposition of silane compounds.
 6. The process as inclaim 1 wherein said etch solution is an ethanolic HF solution.
 7. Theprocess as in claim 5 wherein said ethanolic HF solution comprises about25 percent hydrogen fluoride.
 8. The process as in claim 1 wherein acurrent density in the range of about 162 mA/cm² to 404 mA/cm² isapplied through said silicon structure during said secondelectrochemical anodisation step.
 9. The process of claim 1 wherein saidsilicon structure has resistivity values in the range of about 0.001 to0.0035 ohms-cm.
 10. The process of claim 8 wherein an electric currentdensity in the range of about 162 mA/cm² to 404 mA/cm² is appliedthrough said silicon structure during said first electrochemicalanodisation step.
 11. The process as in claim 1 wherein said double-tanketch chamber defines a cathode region and an anode region isolatedelectrically from each other by said wafer and a seal.
 12. The processas in claim 1 wherein said seal is a fluoroelastomer gasket.